Question about buying a used car?

I am currently looking for a used car.. I dont want to go over $6000 for it. I did find a used 2005 Dodge Neon that I was going to buy for right at $5000. The milage was 93,000 and it looked nice. I took the car to my mechanic so he could tell me if i ws a good choice. When he opened the hood of the car,the heater hose was bypassed and the transmission was replaced with one from the junk yard( according to my mechanic). Also, every time I turned the sterring wheel, the car would die on me. Im glad I found this out before I signed any papers. What do I need to look for in buying a used car. I plan on buying a brand new car when I graduate from college in two years.